/Engine/Plugins/Media/ImgMedia/Source/OpenExrWrapper/Public/OpenExrWrapper.h
Add any attributes and AddChannel after construction. Then you can call CreateOutputFile. Then you can call AddFrameBufferChannel for each channel, and then SetFrameBuffer. After that, you can write out data to the file.
class FTiledOutputFile : public [FBaseOutputFile](API\Plugins\OpenExrWrapper\FBaseOutputFile)